Process of making paper pulp



Sept. l5, 1936. H. LUNDIN ET Al. 2,054,727`

PROCESS OF MAKING PAPER PULP Filed Feb. e, 1955 Solal/zon Slaqye BY i v @04P/m Rm,

Patented Sept. l5, 1936 UNITED STATES 2,054,727 PROCESS 0F MAKING PAPER PULP Harold Lundin and William H. Bitner, Springfield, Ohio Application February 8, 1935, Serial No. 5,659

17 Claims.

The present invention provides a simple and economical process for treating the so-called black liquor produced in the sulphate process of making paper for recovering valuable by-products from such black liquor.

In the sulphate process for producing paper pulp, the wood from which the pulp is made is subjected to digestion with a solution termed .white liquor, which is high in alkali compounds, and it is recognized that an increased strength of the paper pulp iibre is obtained when the digesting liquor is high in its content of sodium sulphide; and it is desirable, therefore, to maintain high concentrations of sodium sulphide in this digesting liquor.

During the digestion operation, the liquor exftracts from the wood various organic materials, such as lignin and various tarry substances, resulting in the formation of what is known as black liquor which is withdrawn from the digestion system.

This black liquor is subjected to treatment for reconverting it into the white liquor for use in further digestion operations. The treatment of the black liquor for its conversion into white liquor comprises spraying it into the shaft of a salt recovery furnace, wherein the `water in the liquor is volatilized and the organic matter is burned, giving off heat which is conveniently utilized in the production of steam. During this operation, the sodium sulphide present in the liquor has a tendency to be oxidized to sodium carbonate and sulphur dioxide, which oxidation obviously substantially depletes the sodium sulphide content in the treated product.

This treated product is the material used in the production of the white liquor for further digestion purposes.

Since the operations of the process are cyclic in character, it is evident that no sulphate pulp, that is, pulp produced by digesting with white liquor high in sodium sulphide can be produced unless salts are withdrawn from thewhite liquor so that sodium sulphate can be added to the salt recovery furnace, as the sodium sulphide content of the White liquor is produced by the reduction of sodium sulphate by the organic matter in the materials being treated in the furnace. It may be noted in this connection, as has been pointed out above, that an increase in the sodium sulphide concentration produces a strong bre, whereas an excess of caustic soda will result in the production of a weak fibre. The best results are obtained when the white liquor contains in the neighborhood oi 35 to 40% of the active alkali as sodium sulphide.

Since the volume of liquor and the amount of salts contained in the black liquor and white liquor in circulation must be kept in balance, it is evident that if the paper pulp plant completely eliminated salt losses, it would be impossible to charge any sodium sulphate to the recovery furnaces, with the result that the white liquor very soon would contain practically no sodium sulphide. In order to make sulphate pulp, it is necessary, therefore, to have either a natural loss of salt in the plant, or to establish these losses in an artificial way by bleeding solution from the plant; and it is obvious that if, in the latter case, the solution thus removed can be treated to obtain by-products of value, or to obtain materials which may beutilized in cyclic operations for replenishing the salt content of the digestion liquor, substantial economies in operation will be effected. It may be noted in this connection that the salt losses in paper pulp plants are in the neighborhood of 15%, these losses being made up by adding sodium sulphate to the recovery furnaces, which sodium sulphate is reduced to sodium sulphide, as pointed out above.

With the above considerations in mind, it may be said that one of the objects of the present invention is to provide a process wherein there is assured a high concentration of sodium sulphide in the white liquor, for the accomplishment of which suilcient quantities of the liquor prepared from the solution of the salts obtained from the salt recovery furnaces is Withdrawn and converted into valuable by-products, all of which may be marketed as such,A or which may be employed in other types of paper-making operations which are commonly associated together in paper-making plants.

A further object of the invention is to provide a process as above-indicated, which is cyclic in character and in which the various reactants are recovered for reuse in the process.

Further objects and advantages of the present process will become apparent as the description proceeds, and the features of novelty will be specically pointed out in the appended claims.

Reference is now made to the accompanying drawing which illustrates a. diagrammatic ow sheet illustrating' the present process.

By referring to the drawing it will be seen that the black liquor, such as has been described above, is charged into a salt recovery furnace in the usual manner, together with sodium sulphate, this material being in excess over the quantity thereof required to make up for salt losses in the paper pulp plant.

In the furnace, the water present in the black liquor is evaporated in the upper part of the shaft. The organic matter present in the black liquor produces, by virtue of its carbon content, and the heat of its combustion, a reduction of the sodium sulphate to sodium sulphide in accordance with the following reaction:-

vlio

'Ihe carbon monoxide formed burns in the upper regions of the furnace, producing carbon dioxide which leaves the furnace together with the evaporated Water and volatile compounds contained in the black liquor.

The remaining charge is tapped from the furnace as a mixture of molten salts, which mixture contains the sodium sulphide produced in the above reaction, together with sodium carbonate, unconverted sodium sulphate, etc. This mixture of fused salts is tapped into water which dissolves the'alkali salts. The solution at this point may be filtered for the removal of undissolved materials. The sludge from this filtration is discarded or recharged back into the salt recovery furnace.

The solution resulting from this filtration is causticized with lime to convert the sodium carbonate present into sodium hydroxide in accordance with the reaction The precipitated calcium lcarbonate, together with excess lime is filtered from the solution, and the resulting filter cake is passed to a lime kiln Where the calcium oxide is regenerated and then returned to the causticizing agitator for the con` version of further quantities of alkali carbonate to alkali hydroxide.

It will be recalled that an excess of sodium sulphat,` was added to a salt recovery furnace.

Because of this fact, the solution resulting from.

This Withdrawn solution is treated to convertl it into valuable by-products in accordance with the description to be given hereafter. It will be apparent that the present process when used in conjunction with a sulphate paperpulp process, increases the sodium sulphide concentration in the White liquor equivalent to the sodium sulphate added, even if no salt losses should occur in the paper pulp plant.

'I'he white liquor withdrawn from the solution circuit contains sodium sulphide, sodium hydroxidev and unconverted sodium sulphate. This withdrawn liquor is treated in accordance with the present process by heating and agitating with finely divided zinc oxide, which converts the sodium sulphide into sodium hydroxide with the formation of precipitated zinc sulphide. The reaction for this is expressed by the equation As outlined on the drawing, this precipitated Zinc sulphide is filtered off and roasted to reconvert the zinc sulphide into zinc oxide, sulphur dioxide -being driven ofi.

However, this zinc sulphide may be used as a filler for the paper pulp, its use in this connection increasing the opacity and brightness of the finished paper, as well as imparting a preservative action by virtue of the antiseptic or bactericidal properties of the material.

In this connection, it may be noted that where the zinc sulphide is roasted for reconversion into zinc oxide,- it is important to keep the temperature in the roasting furnace as low as possible in order to obtain an active zinc oxide. It has been found in practice that with zinc oxide produced at low temperatures, only 25% excess zinc oxide is required over the theoretical amount for the conversion of sodium sulphide to sodium hydroxide.

The filtered solution from the zinc oxide treatment is passed to an evaporator which is deslrably combined with a salt filter. It will be borne in mind that this solution at this point contains principally sodium hydroxide and sodium sulphate, and as the evaporation of the solution proceeds, the sodium sulphate present in the solution crystallizes, and is removed by the filter. 'Ihe recovered sodium sulphate may be returned to the salt recovery furnace for use in treatment with further quantities of black liquor.

In accordance with the present process, it has been found in practice that the reduction of sodium sulphate in a furnace built in accordance with modern principles provided with nozzles blowing air into the charge, that 5,350 B. t. u. are required per pound of mixture of sodium sulphate and coal or carbonaceous matter charged into the furnace to reduce the sodium sulphate charged therein. The solids (carbonaceous compounds together with inorganic salts) in the black liquor of paper pulp plants has an average heat Value of 6,200 B. t. u. per pound, or more than suflicient to reduce one pound of sodium sulphate even if all the salts in the black liquor were sodium sulphate. However, in accordance with the usual practice, only 35% of the salts in the black liquor of paper pulp plants is sodium sulphate, including the sodium sulphate which is added to the furnace, and 65% of the salts are present as carbonate and other salts. As the sodium carbonate and other salts have only to be heated and fused in the furnace, it is evident that the 6,200 B. t. u. which are available are far more than are required to reduce the sodium sulphate present. 'I'herefore, considerably more sodium sulphate can be charged in the salt recovery furnaces in a paper pulp plant Without interfering with the satisfactory operation of the furnaces. The present process takes advantage of the excess carbonaceous material present in the black liquor by adding a considerable excess of sodium sulphate to the charge introduced into the salt recovery furnaces above the amount required to replace any salt loss that may occur in the plant. In accordance with the present process, there is withdrawn from the digestion cycle a quantity of white liquor corresponding to the excess of sodium sulphate added to the furnaces, from which withdrawn white liquor are manufactured caustic soda and sulphur dioxide or zinc sulphide from the caustic carbonate and caustic sulphide contained in the liquor. As previously mentioned, the result of adding an excess of sodium sulphate to the furnaces increases the sodium sulphide concentration in the White liquor Without the necessity of salt losses in the paper pulp plant.

The caustic soda solution produced in accordance with this process may be sold .on the market ,the precipitated zinc sulphide resulting fromI the agitation of the withdrawn white liquor with zinc oxide produces an evolution of sulphur dioxide. I'his step is of advantage, frequently, because a plant manufacturing paper pulp according to the sulphate process, very often has a sulphite pulp plant adjacent to the sulphate plant. The sulphite process makes paper pulp by digesting wood with a solution of calcium bisulphite. The sulphur dioxide produced by the roasting of the zinc sulphide in the present process, may then be used directly in the adjacent sulphite plant, thereby reducing the consumption of sulphur or pyrites which is usually necessarily purchased for the production of the required amounts of active form of zinc oxide. However, the choice of i operation should be the result of circumstances, as the two reactions may be accomplished together by using an excess of zinc oxide.

In the operation of the salt recovery furnace in paper pulp plants, it has been found that an excess of carbonaceous .matter must be present in order to get high yields of sodium sulphide from the sodium sulphate charged to the furnace. Ad-

. dition of more air to the shaft of the furnace to produce more eiiicient combustion usually results in the formation of sodium carbonate as well as sodium sulphide.

When the present process for the manufacture of caustic soda is utilizedin conjunction with a vpaper pulp plant, this is no serious problem, for

the sodium carbonate is converted into' sodium hydroxide as well as the sodium sulphide in the portion of white liquor withdrawn from the paper plant cycle. All that is necessary is that enough sodium sulphide be formed to bring its concentration in the neighborhood of 35 to 40% of the active alkali contained. Sumcient sodium carbonate is converted to sodium hydroxide with lime to bring the caustic soda concentration to the required concentration for the ecient digestion of the wood. A portion of the white liquor is then withdrawn, further causticiaed with lime and the sodium` sulphide converted to sodium hydroxide with zinc oxide.

From the above considerations, it is apparent that the process of the present invention is not limited necessarily to the precise steps herein specifically described and illustrated, but it will be apparent that the details of operation may be modified in various ways as determined by the particular'conditions of operation, without departing from the inventive concept; and it will be understood, therefore, that it is intended and desired to embrace within the scope of this invention such modifications and changes as may be necessary to adapt it to varying conditions and uses.

i 1. A process for converting sodium sulphide into sodium hydroxide, which comprises reducing sodium sulphate with a carbonaceous reducing agent, converting the resulting sodium sulphide to sodium hydroxide by acting upon the said sodium sulphide with zinc oxide, thereby producing zinc sulphide, roasting the resulting zinc sulphide to regenerate the zinc oxide with production of sulphur dioxide, and recovering the sodium hydroxide solution resulting from the treatment with the zinc oxide.

2. A process of making paper pulp which comprises digesting comminuted wood with an alkali digestion liquor high in alkali sulphides to produce a paper pulp of high strength ber with a formation of black liquor, charging the black liquor and a substantial excess of alkali sulphates vinto a smelting furnace, dissolving the resulting melt to form digestion liquor of high alkali sulphide content, separating a quantity of the digestion liquor corresponding to the excess of alkali sulphates added to the smelting furnace, retuming the remainder of the high sulphide content digestion vliquor to digest further quantities of wood, and converting the withdrawn liquor to caustic alkali.

3. A process of making paper pulp which comprises digesting comminuted wood with an alkali digestion liquor high in alkali sulphides to produce a paper pulp of high strength fiber with accompanying formation of black liquor, charging the black liquor together with alkali sulphate into a smelting furnace, the said alkali sulphate being in quantities substantially in excess over normal salt losses, dissolving the melt resulting from the furnace smelting, causticizing the resulting solution to re-form the digestion liquor, the said digestion liquor being high in alkali sulphide, separating a quantity of the said liquor corresponding to the excess of alkali sulphate charged into the smelting furnace, returning the remainder of the high sulphide content digestion Yliquor to digest further quantities of. wood, and

l treating the withdrawn liquor with zinc oxide to convert the said liquor into caustic alkali.

4. A process of making paper pulp which comprises digesting comminuted wood with an alkali digestion liquor high in sodium sulphide to produce a paper pulp of high strength fiber with accompanying formation of black liquor, smelting the black liquor in the presence of sodium sulphate in quantities substantially in excess over normal salt losses, dissolving the resulting smelted product high in sodium sulphid-e, causticizing the resulting solution to re-form the digestion liquor which is high in sodium sulphide, separating a quantity of the said liquor corresponding to the excess of sodium sulphate smelted with the black liquor, returning the remainder of the high sodium sulphide digestion liquor to digest further quantities of wood, treating the withdrawn liquor with suflicient zinc oxide to convert the alkali content of the liquor into caustic alkali with the production of zinc sulphide, separating the zinc sulphide from the caustic alkali solution, and roasting the zinc sulphide to regenerate zinc oxide for treating further quantities of withdrawn liquor.

5. A process of making paper pulp which comprises digesting comminuted wood with an alkali digestion liquor high in sodium sulphide to produce a paper pulprof high strength ber with accompanying formation of black liquor, smelting the black liquor in the presence of` sodium sulphate in quantities substantially in excess of normal salt losses, dissolving the resulting smelted product high in sodium sulphide, causticizing the resulting solution to regenerate the digestion liquor which is high in sodium sulphide, separating a quantity ofthe said liquor corresponding to the excess of sodium sulphate smelted with the black liquor, returning the remainder of the high sodium sulphide digestion liquo-r to digest further quantities of wood, treating the withdrawn liquor with sufficient zinc oxide to convert the alkali content of the liquor into caustic alkali with the production of zinc sulphide, separating the zinc sulphide from the caustic alkali solution, and roasting the zinc sulphide at comparatively low temperatures to regenerate zinc oxide in active form for treating further quantities of withdrawn liquor.

6. A process for producing sodium sulphide which comprises charging predetermined amounts vof sodium sulphate and organic matter into a smelting furnace, smelting the resulting charge to reduce the sodium sulphate to sodium sulphide together with a certain quantity of sodium carbonate, reacting upon the resulting products with lime to convert the sodium carbonate to sodium hydroxide, withdrawing a portion of the resulting mixture containing sodium sulphide, reacting upon this withdrawn sodium sulphide mixture with zinc oxide to produce sodium hydroxide and zinc sulphide, roasting the zinc sulphide to regenerate zinc oxide with the formation of sulphur dioxide, and recovering unconverted sodium sulphate for recharging into the smelting furnace.

'7. A process for producing sodium sulphide which comprises charging predetermined amounts of sodium sulphate and organic matter into a smelting furnace, smelting the resulting charge to reduce the sodium sulphate to sodium sulphide together with a certain quantity of sodium carbonate, causticizing the resulting products to convert the sodium carbonate to sodium hydroxide while leaving the sodium sulphide unchanged, withdrawing a portion of the resulting solution containing sodium sulphide, reacting upon this withdrawn sodium sulphide solution with zinc oxide to produce sodium hydroxide and zinc sulphide, roasting the zinc sulphide at comparatively low temperatures to regenerate active zinc oxide for reuse together with sulphur dioxide, and crystallizing unconverted sodium sulphate for recharging into the smelting furnace.

8. A process for increasing the sodium sulphide concentration of White liquor used in the manufacture of paper pulp, comprising charging a substantial excess of sodium sulphate and organic material into a smelting furnace, smelting the charge to reduce the sodium sulphate to sodium sulphide, dissolving the smelted charge to form green liquor, withdrawing a quantity of the green liquor corresponding to the above-mentioned excess of sodium sulphate, reacting -upon the Withdrawn green liquor with Zinc oxide thereby converting the sodium sulphide thereof into sodium hydroxide with the formation of zinc sulphide, removing the zinc sulphide from the sodium hydroxide, and crystallizing unconverted so-v dium sulphate from the residual solution.

9. A process of making paper pulp which comprises digesting comminuted wood with an alkali digestion liquor high in alkali sulphides to produce a paper pulp of high strength ber with formation of black liquor, separating the black liquor from the pulp, smelting the black liquor together with alkali sulphate to reduce the alkali sulphate to sulphide, dissolving the resulting smelted product in water to regenerate the digestion liquor high in alkali sulphides, withdrawing a quantity of the solution corresponding to the excess of alkali sulphate added, and converting the alkali sulphide in the withdrawn portion into alkali hydroxide.

10. A process for producing alkali hydroxides, which comprses charging a mixture of alkali sulphates and a carbonaceous reducing agent into a smelting furnace to produce a melt of alkali sulphides, leaching the resulting melt to form a solution of the said sulphides, interacting the resulting sulphide solution with a sulphur displacing reagent forming an insoluble sulphide and a solution of alkali hydroxide, crystallizing residual alkali sulphate from the said hydroxide solution, returning the crystallized alkali sulphate back to the furnace, and recovering the alkali hydroxide from the said solution.

11. A process for producing alkali hydroxides, which comprises charging a mixture of alkali sulphates and a carbonaceous reducing agent into a smelting furnace to produce a melt of alkali sulphides, leaching the resulting melt to form a solution of the said sulphides, interacting the resulting sulphide solution with a sulphur displacing reagent forming an insoluble sulphide and a solution of alkali hydroxide, cyclically regenerating and reusing the said'sulphur displacing reagent on further quantities of sulphide solution, and recovering the alkali hydroxide from the solution thereof.

12. A process for producing alkali hydroxides, which comprises charging a mixture of alkali sulphates anda carbonaceous reducing agent into a smelting furnace to produce a melt of alkali sulphides, leaching the resulting melt to form a solution of the said alkali sulphides,l interacting the resulting sulphide solution with zinc oxide, thereby forming insoluble zinc sulphide and a solution of alkali hydroxide, separating the zinc sulphide from the alkali hydroxide solution, roasting the zinc suphide to regenerate zinc oxide, and reusing the regenerated zinc oxide for treating further quantities of the said alkali sulphide solution.

13. A process for converting alkali sulphates into alkali hydroxides, which comprises reducing the alkali sulphates to alkali sulphides and carbonates, leaching the resulting materials with water to form a solution of the said sulphides and carbonates, interacting the resulting solution with a sulphur displacing reagent for forming an insoluble sulphide and hydroxide of the alkali in solution, interacting the resulting solution with lime to form insoluble calcium carbonate and hydroxide of the alkali solution, separating the resulting insoluble sulphide and calcium carbonate from the alkali hydroxide solution, and reconverting the sulphur replacing reagent to oxide and calcium carbonate to lime by heating.

14. A process for converting alkali sulphides into alkali hydroxides, which comprises reducing alkali sulphate with a carbonaceous reducing agent, converting the resulting alkali sulphide to alkali hydroxide Aby acting upon the said alkali sulphide with zinc oxide, thereby producing zinc sulphide, roasting the, resulting zinc sulphide to regenerate the zinc oxide with production of sulphur dioxide, and recovering the alkali hydroxide solution resulting from the treatment with the zinc oxide.

15. A process of making paper pulp, which comprises digesting comminuted wood with an alkali digestion liquor high in alkali sulphides to produce a paper pulp of high strength fiber with formation of black liquor, separating the black liquor from the pulp, smelting the black liquor.A

together with excess alkali sulphate to reduce the alkali sulphate to sulphide, dissolving the resulting smelted product in Water to regenerate the digestion liquor high in alkali sulphides, withdrawing a quantity of the solution corresponding to the excess of alkali sulphate added, and converting the alkali sulphide in the withdrawn portion into .alkali hydroxide, and returning the hydroxide solution to the digestion stage. y

16. A process of making paper pulp, which comprises digesting comminuted wood with an alkali digestion liquor high in alkali sulphides to produce a paper pulp of high strength ber with formation of black liquor, separating the black liquor from the pulp, smelting the black liquor together. with alkali sulphate to reduce the alkali sulphate to sulphide, dissolving the resulting smelted product in water to regenerate the digestion liquor high in alkali sulphides, adding zinc oxide to the resulting liquor to produce alkali hydroxide solution together with zinc sulphide, and returning the alkali hydroxide solution and the zinc sulphide to the digestion stage thereby producing a paper pulp intermixed with zinc sulphide.

17.y A process of making paper pulp which comprises digesting comminuted wood with an alkali digestion liquor high in alkali sulphides and containing substantial quantities of zinc sulphide, and separating the resulting treated pulp inter- `mixed with zinc sulphide from the resulting black liquor.

HAROLD LUNDIN. WILLIAM H. BI'INER. 

